TTY, text telephony for the deaf, allows hearing impaired people to communicate over Public Switched Telephone Networks (PSTN). Some systems, such as those in the United States, use Frequency Shift Key (FSK) modulation of the tones on the line at relatively slow signaling rates, usually 45 baud, to signal character data. The character data may be encoded in one of many schemes, a common one is Baudot characters. The character data is framed by a start bit and a stop bit, resulting in a 7-bit string for each to character.
The performance of TTY systems in the US is mandated by government regulations for all PSTN networks. Generally, the performance criteria are set out in a character error rate. PSTN networks have to have a limited number of errors per a set time period to stay in compliance with the government regulations.
With the advent of voice over data networks systems, TTY services may suffer. Voice over data network systems typically comprise an ingress device, referred to here as a gateway, that ‘translates’ the incoming tones of a PSTN phone call from a caller into a data stream that is then packetized and transmitted as data across a data network. An egress device, referred to here as an egress gateway, then re-modulates the data stream into PSTN tones and transmits it across the PSTN to the called party.
Voice over data networks calls will be referred to here as Voice over Internet Protocol (VoIP) for ease of discussion, with no intention of limiting voice over data networks specifically to IP networks. Similarly, while the ingress and egress devices will be referred to here as gateways, no limitation to networks complying with the International Telecommunications Union (ITU) recommendations for multimedia communications systems, such as H.323, is intended.
Data loss and encoding schemes in VoIP networks may result in higher than acceptable error rates for TTY data streams. The higher error rates may result in VoIP systems being noncompliant with government regulations for TTY systems.